How do I import export .nif
Here's a link to an addon:

http://niftools.sourceforge.net/forum/viewtopic.php?f=13&amp;t=5823

To install it:


Download the folder
Open User Preferences (Ctrl+Alt+U) and under Add-ons, click Install from File. Then navigate to the file and select it.
It should now appear in the window and you can tick the checkbox in the upper right to enable it.


Related: Installing an addon